Formed fiber containers, such as paper plates and trays, are commonly produced either by molding fibers from a pulp slurry into the desired form of the container or by pressing a paperboard blank between forming dies into the desired shape.
Pressed paperboard containers may be made as noted in one or more of U.S. Pat. Nos. 4,606,496; 4,609,140; 4,721,499; 4,721,500; 5,203,491; 6,715,630; and United States Patent Application Publication No. 2006/0208054 (pending as U.S. patent application Ser. No. 10/963,686). Equipment and methods for making paperboard containers are also disclosed in U.S. Pat. Nos. 4,781,566; 4,832,676; 5,249,946 and 4,588,539. U.S. Pat. Nos. 6,186,394 and 6,039,682 disclose composite paperboard containers having embossed surfaces.
Pulp molded articles, after drying, are strong and rigid but generally have rough surface characteristics and generally contain far more fiber than pressed paperboard plates. They are not usually coated and are susceptible to penetration by water, oil and other liquids. Pressed paperboard containers, on the other hand, can be decorated and coated with a liquid-resistant coating before being stamped by the forming dies into the desired shape. Pressed paperboard containers generally contain far less fiber and cost less, requiring less storage space than the molded pulp articles. Large numbers of paper plates and similar products are produced by each of these methods every year at relatively low unit cost. These products come in many different shapes, rectangular or polygonal as well as round, and in multi-compartment configurations.
Primarily, due to the presence of pleats, even well pressed paperboard containers have tended to exhibit somewhat less strength and rigidity than do comparable containers made by the pulp molding processes. Much of the strength and resistance to bending of a plate-like container made by either process lies in the sidewall and rim areas surrounding the center or bottom portion of the container. When in use, such containers are often supported by the rim and sidewall while the weight held by the container is located on the bottom portion. Thus, the rim and sidewall generally are placed in tension and flexure when the container is being used.
In plate-like structures made by the pulp molding process, the sidewall and overturned rim of the plate are unitary, cohesive fibrous structures which have considerable resistance to bending as long as they are not damaged or split. Because the rim and sidewall of the pulp molded containers are of a cohesive, unitary structure, they may be placed under considerable tension and flexure without failing. Plates produced by the pulp molding process do not typically have a continuous functional coating to prevent strength loss during use with hot, moist foods. Internal chemicals can be used to retard moisture and grease absorption. For improved moisture resistance, a secondary film can be laminated to the plate in a separate, post formation, step resulting in a significantly higher cost.
In contrast, when a container is made by pressing a paperboard blank, the flat blank must be distorted and changed in shape and area in order to form the blank into the desired three dimensional shape. This necessary distortion results in seams or pleats in the sidewall and rim, the areas of the container which are drawn in toward the center in press forming the container resulting in a decrease in the circumference of the formed container as compared to the blank. Unless considerable care is employed during the process of pressing, these seams or pleats can constitute material lines of weakness in the sidewall and rim areas about which such containers tend to bend more readily than do containers having unpleated sidewalls and rims. Moreover, such seams or pleats will often have a tendency to open or unfold under tension or flexure as if attempting to return to their original flat shape, particularly if exposed to moisture, or even worse, moisture at elevated temperatures. The necessary location of these pleats in the sidewall and rim of pressed paperboard containers places the greatest weakness in the area requiring the greatest strength. Unless carefully formed, such containers have typically have been unable to support loads comparable to pulp molded containers of equivalent fiber content. Under tension, flexure or torsion, pleats exhibit a tendency to open and/or hinge. Accordingly, most known pressed paperboard containers typically have significantly less load carrying ability than do pulp molded containers unless particular care is employed to transform disrupted regions in the plates into substantially integrated fibrous structures during the pressing process. In contrast to pulp molded plates, the pressed containers can easily have a continuous functional coating applied to the paperboard prior to forming, resulting in enhanced performance with hot and moist foods. Being less costly than an equivalent pulp molded plate, a pressed paperboard plate with enhanced strength and Rigidity as well as a better moisture barrier would have significant commercial value.
Further, pressed paperboard plates typically have relatively poor insulation properties as a result of their thinner material construction. Consequently, the bottom of the plate can get warm when hot food is placed on top. Carrying hot food can be uncomfortable for the user of the plate.
Many efforts have been made to strengthen pressed paperboard containers while accommodating the necessary reduction in area at the sidewalls and rims. Blanks from which paperboard containers are pressed have been provided with score lines at their periphery to eliminate the random creation of seams or pleats. The score lines are typically provided in a manner that results in internal delamination of the scored areas of the blank, thereby causing the pleats to form in the scored areas but generally, at least according to conventional wisdom, leading to plates with slightly lower strength than equivalent plates with random pleats. Scores can be created either on the top side or the bottom side of a blank. The score lines thus define the locations of the seams or pleats. As alluded to before, efforts have been made in pressing the pleats to reform the disrupted regions caused by internal delamination attendant upon formation of a plate in order to improve Rigidity. While substantial reforming is possible, it is commonly less than ideal in most real world manufacturing processes as obtaining the best results requires considerable care in selecting the appropriate contours for the dies, maintaining the dies in alignment, ensuring that the board is moisturized to the appropriate levels and temperatures are maintained within the desired ranges as well as assuming that sufficient pressure is applied to reform the bonds in the descriptive regions. Unfortunately, it has not proved trivial to greatly increase the strength of pressed paperboard plates beyond that attainable with 230 pound board, by merely increasing the basis weight of the paperboard blank from which they are formed as the difficulty of forming well integrated pleats seems to increase with the caliper of the blank.
Various methods of making pressed paper articles are known in the art. For example, U.S. Pat. No. 860,385 discloses a method of making paper tube caps from multiple layers of paper. The meeting faces of the paper layers have glue applied thereto, and the compound multilayered paper blank is formed in a die before the glue sets.
U.S. Pat. No. 2,231,345 discloses a method of making multi-ply trays from paper stock and wood. The layers are pressed together in such a way as to form corrugations in the paper in the corners of the tray so as to offset the tendency of the paper to wrinkle.
Pressed paperboard products can be fabricated from a single thick layer of paperboard. However, one reason the pressed paper plates are often weaker than pulp molded plates lies in the basis weight range which can most easily be formed into plates. Thick layers of board are more difficult to pleat and form properly than one or multiple thinner layers. Thus, one way that has been attempted to fabricate stronger paper products is assembling two or more layers of paper and/or other sheet material.
Prior art methods often employ interleaving for securing multiple layers of paper or other material. Referring to FIGS. 1 and 2, for example, a layered structure 1 containing an upper paper layer 2 and a bottom paper layer 3 is shown. The upper paperboard layer 2 has an upper surface 2a and a lower surface 2b. The lower paperboard layer 3 has an upper surface 3a and a lower surface 3b. Typically, one or both layers is not shape-sustaining. A non shape-sustaining sheet of material will sag or droop under its own weight, for example, if a plate sized blank is held only at one edge even if a slight downward bow is applied transversely to the bending moment in the web as will be appreciated by one of skill in the art.
Optionally, a layer of adhesive 4 between the lower surface 2b of the upper layer and upper surface 3a of the lower layer secures the paperboard layers 2 and 3 in a fixed relative position prior to forming.
When formed into a pleated configuration as shown in FIG. 2, pleat 5 is formed into an interleaved configuration because the upper paperboard layer 2 and the lower paperboard layer 3 do not pleat independently of each other. As can be seen, lower paperboard layer 3 has one or more sinuous (S- or Z-shaped) pleated portions or folds 3c and/or 3d, and upper paperboard layer 2 has one or more sinuous folds 2c and/or 2d. However, it can be seen that these folds are vertically disposed one above the other in the layers. Thus, the sinuous folds in pleated portions 3c and 3d of the lower paperboard layer 3 are directly above the respective sinuous folds in the pleated portions 2c and 2d of the upper paperboard layer 2. For ease of reference, we term this form of pleating “interleaved pleating.” Interleaved pleating (with or without adhesive) is shown, for example, in U.S. Pat. Nos. 5,203,491 and 5,120,382. Typically, a pleat will consist of one or two sinuous regions with pleats comprising a Z-shaped region next to an S-shaped region being preferred and referred to as U-shaped pleats or omega-shaped pleats depending upon the relative positions of the S-shaped and Z-shaped regions. In our experience, when two layers of board are used in plate making, interleaved pleats provide little benefit over use of a single layer of comparable thickness.
There yet remains a problem in that a single thick layer of paperboard is more difficult to form and pleat properly than one or more multiple thin layers. However, interleaved pleats of multilayered paper products can result in pronounced lines of weakness which can open or hinge during use carrying food, or other loads, or from handling or flexing.
What is needed is a method for making multi-ply products, particularly paper products, which avoids these difficulties.